The present invention relates generally to optical cables that include a strand element consisting of several light waveguides for the transmission of optical signals or light waves. Specifically, this invention relates to a filling compound for light waveguide leads or optical waveguide buffer tube and/or light or optical waveguide cables.
Typically, several light waves guides leads, for the transmission of optical signals or light waves, are disposed within a protective casing defining a light waveguide cable. Each light waveguide lead typically consist of a coated fiber surrounded by a sheath. As disclosed in U.S. Pat. No. 4,331,379 the space between the light waveguide leads and the protective casing can be filled by a compound that may consist of a thixotropic additive or oil. The space between the fibers and the protective sheath in the light waveguide lead may also be filled by a filling compound.
Prior filling compounds have not been entirely successful. This is due, in part, to the fact that requirements of the filling compounds are, in part, contradictory. It is desirable for a filling compound to exhibit the following properties:
(a) The filling compound must retain its properties over a wide temperature range, for example from -40.degree. C. to +70.degree. C.; accordingly, the filling compound should not exhibit an excessive hardness at low temperatures that will cause a mechanical stressing of the light waveguides due to an excessive increase of the viscosity nor should the filling compound exhibit a phase separation of the fluid phase from the solid phase (i.e., a drip-out from the cable) at high temperatures, PA0 (b) the filling compound should have a low viscosity at room temperature in its unprocessed condition in order to enable the filling of cables during the cable manufacture operation even through cables and leads having a small inside diameter and at low excess pressure, PA0 (c) the filling compound should avoid the build-up of tensile or compressive forces in the light waveguide leads as much as possible, an increase tensile or compressive force will cause the light waveguide leads to attenuate, PA0 (d) the filling compound should exhibit an optimally low moisture absorption tendency, PA0 (e) the filling compound should be compatible with and not exhibit a detrimental effect on any of the materials it will contact, i.e., the protective sheath, casing, and the fiber coating, PA0 (f) the filling compound should be resistant to aging, both in a chemical and in a physical sense.
It is known that fumed silica compounds have silanol groups on their surface that have a tendency to bond with adjacent particles of the matrix via weak hydrogen bondings. When using thixotropic oils (fumed silica+oils) as filling compounds, the weak hydrogen bondings are broken, at low energy influences; the fumed silica separates from the matrix. The separation of the fumed silica from the matrix results in a substantial decrease in viscosity. The separation of the fluid phase (oil) from the solid phase (fillers) is further intensified when the filling compound is used to fill the optical waveguide buffer tube because of the capillary forces in the optical waveguide buffer tube. As a result, the fluid phase easily drips out. This further results in the remaining portion of the filler compound having a greater viscosity than the original filling compound causing it to exert a pressure on the optical waveguide buffer, thereby causing the fiber to attentuate. Because of these contradictory demands on the filling compound, e.g. the requirement that the filling compound have a solid consistency on the one hand in order to prevent drip-out and on the other hand have a low-viscosity in order to keep the mechanical stressing of the light waveguides and, thus, the attenuation of the fibers, low--most filling compounds have not functioned satisfactory in light waveguide leads and cables. Accordingly, there is a need for an improved filling compound for light waveguide leads and cables.